1. Field of the Invention
The present invention relates to a method for manufacturing a recording original disc for optical information recording media (optical discs such as video discs, compact discs, etc., for example), and particularly to a method for manufacturing a recording original disc which is suitable to enhance recording density.
2. Description of Related Art
First, a conventional method for manufacturing optical discs will be described. The optical discs are manufactured through a recording original disc manufacturing process, an electroforming process, and a replication process as described below.
a. Recording original disc manufacturing process
In this process, a recording original disc which is used to produce a stamper having a surface shape to be transferred onto optical discs is manufactured.
First, a glass substrate is sufficiently cleaned, then silane coupling agent is vaporized to be adsorbed by the glass substrate so that a photoresist layer is well formed without being peeled off in a next process, that is, the glass substrate is subjected to an adhesion agent treatment. After being dried, photoresist is uniformly coated on the glass substrate by a spin coat method, and then the photoresist layer is exposed to light by a laser cutting method to form a latent image of recording pits. Subsequently, developer is dropped onto the glass substrate while rotating the glass substrate to develop the latent image of the photoresist layer, thereby obtaining a recording original disc.
b. Electroforming process
In this process, a stamper is manufactured by using the recording original disc.
First, in order to provide the recording original disc with conductivity, a Ni thin film having a thickness of about 100 nm is formed on the surface of the recording original disc by a Ni sputtering method, and then subjected to an electroplating treatment in nickel sulfamate bath while the Ni thin film is used as a cathode and depolarized nickel having high solution efficiency is used as an anode. The electroplated Ni plate and the Ni thin film are exfoliated from the recording original disc, and the photoresist adhering to the surface of the recording original disc is removed to obtain a master disc. The master disc may be used as a master stamper by polishing the back surface thereof. However, it is normally adopted that the oxidation treatment and the electroforming treatment are repeatedly performed on the surface layer of the master disc to form a mother disc, and a stamper is manufactured by using the mother disc. With this operation, about twenty five stampers are manufactured from one master disc.
c. Replication process
In this process, an optical disc substrate having recording pits is manufactured by using the stamper.
First, a resin pellet for a substrate is sufficiently dried. A stamper is secured to a movable mold of an injection molding machine, and a heated and melted resin for the substrate is injected into a cavity between the stamper and a fixed mold having a mirror-polished surface. The injected resin is compressed and kept in pressure, and then forcedly cooled to form an optical disc substrate. Thereafter, the optical disc substrate thus formed is removed from the molding machine.
FIG. 1 is a schematic cross-sectional view showing an optical disc which is manufactured by a conventional method as described above. A metal reflection film 54 and a resin protection film 56 are formed on an optical disc substrate 52 manufactured through the processes a to c as described above, thereby obtaining an optical disc 50.
With the conventional optical disc 50 thus formed, the recording pits (recess portions) 57 of the disc 50 are unintentionally formed to have slant surfaces at the side walls 58 thereof (that is, the side walls of the optical disc are not perpendicular or vertical to the optical disc surface), that is, so-called sag occurs. Therefore, reproduction signals are disturbed due to the sag, and this obstructs the design of the recording pits 57 having high fineness and high recording density.
The cause of occurrence of the sag as described above will be described below.
FIG. 2 is a cross-sectional view showing the recording original disc manufacturing process in the conventional technique as described above. A photoresist layer 64 is formed on a glass substrate 62, and a laser beam L is irradiated to the photoresist layer 64 to perform an exposure process. The photoresist layer 64 thus exposed is developed to form recess portions 66 serving as recording pits as shown in FIG. 2, thereby forming a recording original disc 60. Since the intensity of the laser beam L has a Gaussian distribution, a latent image which is formed on the photoresist layer 64 in the exposure process of the laser beam L also has a shape corresponding to a substantially Gaussian distribution. That is, the sectional shape of a recess portion 66 is changed from a shape indicated by a through shapes indicated by b, c and d to a shape indicated by e with the progress of the development. Accordingly, sag corresponding to a spread of an edge portion of the Gaussian distribution of the laser beam L occurs, and this sag is copied onto the optical disc substrate 52.
In order to solve the "slant surface" problem of the side walls of the recording pits as described above, Japanese Patent Application Laid-open No. 3-108141 and Japanese Patent Application Laid-open No. 4-248145 have proposed techniques for forming recess portions of a recording original disc by etching an etching layer through opening portions of a photoresist layer. According to these techniques, in consideration of the fact that if the recess portions are formed by an exposure process using a laser beam, the side walls of the recess portions are designed to be slanted because the intensity of the laser beam has a Gaussian distribution, the recess portions are formed by using an etching process in place of the exposure process of the laser beam to prevent occurrence of slant side walls in the recess portions.
This method will be described with reference to FIGS. 3A to 3C. First, as shown in FIG. 3A, a layer 72 which will be subjected to an etching treatment (hereinafter referred to as "etching-applied layer") and a photoresist layer 74 are formed or laminated on the substrate 70, and the photoresist layer 74 is exposed to a laser beam h.nu. in a desired pattern to form a latent image. Subsequently, the latent image of the photoresist layer 74 is developed to form opening portions 76 corresponding to recording information as shown in FIG. 3B, and then an etching treatment is performed with using the photoresist layer 74 as a mask. Thereafter, the residual photoresist layer 74 is removed to form recess portions 78 in the etching-applied layer 72 as shown in FIG. 3C, thereby obtaining a recording original disc 80. An optical disc is manufactured by using this recording original disc 80.
In the optical disc manufacturing process as described above, it is important that the side walls of the recess portions serving as recording pits are formed in a vertical direction to the surface of the optical disc, i.e. perpendicular to the disc surface, in order to enhance the recording density with keeping an excellent reproduction signal characteristic.
However, in the manufacturing method of the recording original disc shown in FIGS. 3A to 3C, it is difficult to achieve a process which simultaneously satisfies both high vertical anisotropy and high etching selectivity because the photoresist is used as an etching mask. FIG. 4 is a cross-sectional view showing the recording original disc to explain the above-mentioned difficulty. In FIG. 4, if the opening portion 76 is formed in the photoresist layer 74 which is formed on the etching-applied layer 72 on the substrate 70 and the etching treatment using an ion beam I is performed, the shape of the photoresist layer 74 in which the opening portion 76 is formed is gradually degraded with the progress of the etching because sufficient selectivity is not obtained. Another cause of the degradation of the edge portion of the photoresist layer 74 resides in that the edge portion is formed to have a slant surface as described above, and thus it is liable to suffer the impact of the ion beam I. Accordingly, the shape of the opening portion 76 of the photoresist layer 74 and the shape of the recess portion 78 of the etching-applied layer 72 are changed from the shape indicated by a virtual line 801 through the shape indicated by a virtual line 802 to the shape indicated by a virtual line 803 with the progress of the etching treatment. Like this, the side walls 781 of the recess portion 78 are also designed to have slant surfaces, and thus they are not formed vertically to the substrate 70. Therefore, according to the conventional techniques as described above, it is unavoidable that the side walls of the recording pits are formed to be slanted, and thus it is difficult to achieve the high recording density of the optical disc.